Radio frequency (RF) communication devices, such as cellular telephones and the like, typically include a receiver lineup that is coupled to an antenna. The receiver lineup converts RF signals received by the antenna to intermediate frequencies and, ultimately, to baseband frequencies.
The presence of in-band and out-of-band blocker signals and modulated interferers in the area of a receiver can adversely affect the performance of the receiver lineup. Such blockers can compress the receiver and cause receiver noise floor to increase and receiver gain to decrease. Depending on blocker power and frequency, blockers can either compress RF front-end circuits or the IF back-end circuits in the receiver.
In the presence of large blockers, receivers use an Automatic Gain Control mechanism (AGC) to prevent saturation. Traditionally, these AGC schemes have utilized some form of baseband or RF detector that reduces the gain of the receiver in response to blocker presence. A baseband detector can detect both in-band signals and blockers located close to the in-band frequency. The detection bandwidth of baseband detectors are limited by the filtering in the receiver chain, which limits accessible frequency content. Out-of-band blockers and higher frequency in-band blockers are not easily detected in the baseband. Such blockers can compress the RF front-end of the receiver and lead to poor performance if the front-end components are not sufficiently linear.
An RF detector, such as a log amplifier, can detect far out blockers as well as in-band signals. An RF detector is usually placed at the front of the RF chain where there is less filtering and the detection bandwidth the largest. An RF detector gives no indication of the frequency location of the signals it measures, but can be used to reduce the gain of the receiver chain in response to a large detected signal. The main issue with this type of detector is that because it gives no indication of the blocker frequency it is difficult to determine the optimum gain selection for the receiver. For example, if an out of band blocker is input to the receiver, then to prevent the receiver front end circuits from compressing the gain of the RF blocks should be reduced. With this type of detector it is not possible to determine the frequency location of the blocker relative to the desired signal and so it is difficult to determine where to reduce the gain in the receiver chain. Also, most RF detectors have a limited input dynamic range and cannot reliably detect low power signals. These lower power signals, if located close to the band of interest, can be amplified in the RF front-end of the receiver and cause compression of the low frequency receiver back-end.
As supply voltages provided to receiver lineups are reduced in successive process nodes, it becomes more difficult for receiver lineups to meet linearity performance criteria specified by relevant communication standards and, thus, it becomes difficult for a receiver lineup to accommodate the presence of a blocker without becoming compressed. Additionally, receiver lineups that are highly linear generally do not represent optimum designs as they need to be over-designed to meet all linearity requirements over process and temperature.
A known receiver lineup is shown at reference numeral 100 in FIG. 1. The receiver lineup 100 includes a low noise amplifier (LNA) 102, which is coupled to an antenna 104. The LNA 102 is coupled to a mixer 106, the output of which is low pass filtered by a filter 108. Subsequently, the signal from the filter 108 is coupled to a variable gain amplifier 110 and a low pass filter 112, which is further coupled to a variable gain amplifier 114 and a low pass filter 116. The output of the low pass filter 116 is coupled to an analog-to-digital converter (A/D) 118. The A/D 118 transforms the analog signals from the low pass filter 116 to a digital format that may be processed by a digital baseband processor 120. As shown in FIG. 1, the digital baseband processor 120 includes an intermediate frequency (IF) detector 122, which may be implemented using hardware and/or software.
In operation, the signal at the antenna 104, shown at node 130, has characteristics as shown in FIG. 2. That is, an RF signal 202 that is much smaller than the amplitude of a blocker 204, which is an in-band or out-of-band blocker or modulated interferer from another device or from some other source.
As shown in FIG. 3, which represents the characteristics of the signals at node 132, after amplification by the low noise amplifier 102, the RF signal 302 and the blocker 304 are amplified, as is the noise floor 306. As will be readily appreciated, the high amplitude blocker 304, when provided to the mixer 106, creates additional noise because the mixer 106 is driven in a non-linear region of operation called compression. Compression of the mixer also causes its gain to decrease. The mixer compression dramatically increases the noise figure of the receiver lineup 100. This result is shown in FIG. 4, which represents the characteristics of the signals at node 134. As shown in FIG. 4, IF signal 402 and blocker 404 have increased in amplitude, but such an increase is not in proportion to the noise floor 406, which grows disproportionally due to the mixer compression caused by the blocker 304. The blocker cannot be easily eliminated or the mixer compression caused thereby detected because the blocker 304 will be subsequently filtered out before the signals reach the IF detector 122.